I'll bet the Pedros fix gear tool (the Trixie) would work. (It's spanner grabs fix gear lockrings far better than my lesser Park spanner and especially the 12t lockrings where the cog is not there as a shoulder. That headset lockring spanner notch looks a lot like the 12 tooth ones.)

It seems Pedros changed the plate stock of the Trixie to a thinner one. I have only the older ones which are very good tools. (I coveted the second one for years as it hung on a bikeshop display. Set up the Mooney fixed and finally had justification to "liberate" it. (The shop owner expressed surprise that a tool that good marked at $30 would sit there that many years. Was happy to see it go to a good home.)

Ben